Mobilization of isotopically heavy sulfur during serpentinite subduction

Primitive arc magmas are more oxidized and enriched in sulfur-34 (34S) compared to mid-ocean ridge basalts. These findings have been linked to the addition of slab-derived volatiles, particularly sulfate, to arc magmas. However, the oxidation state of sulfur in slab fluids and the mechanisms of sulfur transfer in the slab remain inconclusive. Juxtaposed serpentinite and eclogitic metagabbro from the Voltri Massif (Italy) provide evidence for sulfur mobilization and associated redox processes during infiltration of fluids. Using bulk rock and in situ δ34S measurements, combined with thermodynamic calculations, we document the transfer of bisulfide-dominated, 34S-enriched fluids in equilibrium with serpentinite into adjacent metagabbro. We argue that the process documented in this study is pervasive along the subduction interface and infer that subsequent melting of these reacted slab-mantle interface rocks could produce melts that display the characteristic oxygen fugacity and sulfur isotope signatures of arc magmas worldwide.


Thermodynamic reaction path modelling
Thermodynamic reaction-path calculations were conducted with the EQ3/6 software package from (63) and the Deep Earth Water (DEW) Model (64,65,83) database.

Fig. S-4.
Calculation of the SO2(aq) = H2S(aq) and CO2(aq) = CH4(aq) equilibrium boundaries, and the magnetite-hematite buffer as a function of temperature (for pressures of 1.0, 1.5 and 2.0 GPa).This suggests that the more reduced species (H2S) would coexist with magnetite-bearing serpentinite at the P-T conditions inferred for HP metamorphism and metasomatism of the Voltri Massif (see main text for discussion).The calculations were made using the DEW_2019 version.

Isotope fractionation calculations
Additional isotope fractionation and mixing models are plotted below.For details of the calculation see methods section in the manuscript.All mixing is calculated as: Sfinal,WR = (1-f)* H2Smetasom.+ Sgabbro whereby metasomatic H2S of 3500 µg/g is incrementally added to an eclogitic metagabbro with Ssulfide =1500 µg/g and d 34 Ssulfide = 1.7‰.The metasomatic H2S is either derived directly from the serpentinite or is produced by sulfate reduction either through open or closed system processes.
Different scenarios are shown below.

Fig. S-5
shows the effects of simple sulfur isotope mixing of H2S (metasomatic H2S) that is derived from serpentinite and is mixing with the eclogitic metagabbro during fluid infiltration and metasomatism (compare with Fig. 9C in the main text).

Fig. S-6
shows the effects of S isotope fractionation assuming open system fluid evolution in terms of sulfate availability, i.e., d 34 Smetasom.H2S is produced by the conversion SO4 2-à H2S and S isotope fractionation is only controlled by temperature and the value of the produced metasomatic H2S does not change during reaction progress (no reservoir effects).This process would result in constant d 34 S values of the pyrite rims throughout the transect, however, open system processes are unlikely to take place in subduction zone settings.

Fig. S-5.
Isotope mixing model of H2S (metasomatic H2S) influx from serpentinite and mixing with the eclogitic metagabbro (compare with Fig. 9C in the main text).Only isotope fractionation during pyrite precipitation from H2S is considered, with e = 0.9 at 400°C and 0.7‰ at 500°C.Initial sulfide content of the fluid is 3500 µg/g at variable initial d 34 SH2S, which is incrementally added to the gabbro (Ssulfide = 1500 µg/g, d 34 S= 1.7‰).Addition of metasomatic H2S initiates from fraction (f) = 0 and decreases towards f = 1, with f = 1 representing the S content of the initial gabbro.(A) Sulfide content of the bulk rock, (B) evolution of the d 34 S value of the metasomatic H2S assuming different initial d 34 S values between 0‰ and 25‰, (C) mixing of metasomatic H2S assuming different initial d 34 S values between 0‰ and 25‰ with gabbro containing 1500 µg/g S and d 34 S = 1.7‰.All models were calculated at 500°C.Isotope fractionation model following a closed system fluid evolution, i.e., H2S is produced by closed system sulfate reduction of 3500 µg/g sulfate, which results in a continuous increase in d 34 Smetasom.H2S with d 34 Smetasom.H2S, final = d 34 Ssulfate, initial (compare with Fig. 9D in the main text).The produced H2Smetasom. is incrementally added to a gabbro with a S content of 1500 µg/g and d 34 S = 1.7‰.Addition of metasomatic H2S initiates from fraction (f) = 0 and decreases towards f=1, with f = 1 representing the S content of the initial gabbro.(A) Sulfide contents of bulk rock and metasomatic H2S as a function of the fraction, (B) evolution of the d 34 S values of the formed metasomatic H2S and the remaining sulfate assuming an initial d 34 Ssulfate value of 15‰, (C) evolution of the d 34 S value of the metasomatic H2S assuming different initial d 34 Ssulfate values from 5‰ to 35‰, (D) mixing of metasomatic H2S assuming different initial d 34 Ssulfate values between 5‰ and 35‰ with gabbro containing 1500 µg/g S and d 34 S = 1.7‰.All models were calculated at 400°C.

Fig
Fig. S-1.BSE images showing the sulfide mineralogy of the studied samples with the Co element distribution overlain.Red squares indicate locations of SIMS pits and corresponding d 34 S values (in permil versus V-CDT).(A) Pyrite grains from zone IVa containing abundant silicate inclusions (amphibole, clinozoisite, albite) with some grains showing a distinctive, porous overgrowth rim (pyrim) that is in contact with a serpentine-talc matrix.(B) and (C) Pyrite grain from zone IVb that contains omphacite, glaucophane, and clinozoisite (epidote) inclusions.(D) Pyrite grain from zone IVb containing metamorphic minerals including omphacite, garnet, plagioclase and clinozoisite, (E) Pyrite containing numerous silicate inclusions (quartz + garnet + omphacite) next to garnet and (F) trace amount of chalcopyrite rimmed by amphibole.All silicate mineral inclusions are rimmed by pyrite with higher d 34 S values compared to the inner zones indicating that the 34 S-enriched pyrite formed during or subsequent to subduction metamorphism.

Fig
Fig. S-2.BSE images and element distribution maps of sulfides from samples V18-A03 (zone IVa) and V18-A05 (zone IVb), as well as EMP travers analyses (in C and F) indicated by the red arrows.(A) to (C) Euhedral pyrite with a distinct Co zonation and high Co contents in the grain center.The pyrite is surrounded by a serpentine-talc matrix.(D) to (F), (i) Pyrite with corrosion rim that is characterized by lower S contents and considerable amounts of silicate inclusions (V18-A05).(g) to (h) Pyrite with a corrosion rim that is in contact with serpentine-talc intergrowths filling the surrounding, whereas the core shows an oscillatory zonation in Co (V18-A03).

Fig
Fig. S-3.BSE image (A) and element distribution maps of a pyrite grain from sample V18-A13 showing distinct zonations in Co concentrations (B) and slight variations in Ni contents (C).EMP traverse analyses (black arrow in (A), (B), (C)) allows identifying three zones based on distinct Co (D) and Ni (E) contents: i) an inner, Co-rich zone with up to 3.97 wt.% Co and very low Ni contents (<0.02wt%); ii) a very Co-poor zone (Co<0.2wt%) with up to 0.7 wt.% Ni; and iii) an intermediate Co zone with Co contents around 0.7-0.9wt.% and low Ni contents (<0.02wt%).Note that slight color variations in the BSE image suggests that several pyrite grains grew to one big grain during metasomatism.The in situ d 34 S values are shown in (A) in ‰ versus V-CDT and the location of the SIMS pits is shown in (A) to (C) by the orange squares.

Fig. S- 7
Fig. S-7shows the effects of S isotope fractionation following a closed system fluid evolution during sulfate reduction associated with fluid infiltration.In a closed system the conversion SO4 2- à H2S results in a continuous increase in d 34 Smetasom.H2S with d 34 Smetasom.H2S, final = d 34 Ssulfate, initial (compare with Fig.9Din the main text).This scenario would result in an increase in the d 34 S values of the pyrite rims along the transect into the metagabbro.In this case, the model fits most measured data using d 34 S sulfate values of ~15 to 22‰, with one data point requiring an initial d 34 S sulfate value of ~30‰.

Fig
Fig. S-8shows the effects of S isotope fractionation following a Rayleigh distillation model.During Rayleigh distillation, the produced H2S reaches highly positive d 34 S values as H2S is continuously removed from the system during the conversion SO4 2-à H2Smetasom.H2S.Note, the relatively constant d 34 S values of the pyrite rims throughout the transect and the lack of very high (i.e., >15‰) in situ d 34 S sulfide values point against such a process.
Fig. S-6.Isotope fractionation model assuming open system fluid evolution in terms of sulfate availability, i.e., d 34 Smetasom.H2S is only controlled by temperature and does not change during reaction progress (no reservoir effects).The produced H2Smetasom., formed by sulfate reduction of 3500 µg/g sulfate, is incrementally added to the gabbro (Ssulfide = 1500 µg/g, d 34 S= 1.7‰).Addition of metasomatic H2S initiates from fraction (f) = 0 and decreases towards f = 1, with f = 1 representing the S content of the initial gabbro.(A) Sulfide contents of the bulk rock, (B) evolution of the d 34 S value of the metasomatic H2S assuming different initial d 34 Ssulfate values between 10‰ and 40‰, (C) mixing of metasomatic H2S assuming different initial d 34 Ssulfate values between 10‰ and 40‰ with gabbro containing 1500 µg/g S and d 34 S = 1.7‰.All models were calculated at 400°C.
Fig. S-7.Isotope fractionation model following a closed system fluid evolution, i.e., H2S is produced by closed system sulfate reduction of 3500 µg/g sulfate, which results in a continuous increase in d 34 Smetasom.H2S with d 34 Smetasom.H2S, final = d 34 Ssulfate, initial (compare with Fig.9Din the main text).The produced H2Smetasom. is incrementally added to a gabbro with a S content of 1500 µg/g and d 34 S = 1.7‰.Addition of metasomatic H2S initiates from fraction (f) = 0 and decreases towards f=1, with f = 1 representing the S content of the initial gabbro.(A) Sulfide contents of bulk rock and metasomatic H2S as a function of the fraction, (B) evolution of the d 34 S values of the formed metasomatic H2S and the remaining sulfate assuming an initial d 34 Ssulfate value of 15‰, (C) evolution of the d 34 S value of the metasomatic H2S assuming different initial d 34 Ssulfate values from 5‰ to 35‰, (D) mixing of metasomatic H2S assuming different initial d 34 Ssulfate values between 5‰ and 35‰ with gabbro containing 1500 µg/g S and d 34 S = 1.7‰.All models were calculated at 400°C.

Fig. S- 8 .
Fig. S-8.Isotope fractionation model following a Rayleigh distillation model during sulfate reduction to form H2S (H2Smetasom.).The produced H2Smetasom. is incrementally added to a gabbro with a S content of 1500 µg/g and d 34 S = 1.7‰.Addition of metasomatic H2S initiates from fraction (f) = 0 and decreases towards f=1, with f = 1 representing the S content of the initial gabbro.(A) Sulfide contents of bulk rock and metasomatic H2S as a function of the fraction, (B) evolution of the d 34 S values of the formed metasomatic H2S and the remaining sulfate assuming an initial d 34 Ssulfate value of 15‰, (C) evolution of the d 34 S value of the metasomatic H2S assuming different initial d 34 Ssulfate values from 0‰ to 30‰, (D) mixing of metasomatic H2S assuming different initial d 34 Ssulfate values between 0‰ and 30‰ with gabbro containing 1500 µg/g S and d 34 S = 1.7‰.All models were calculated at 400°C.